Highly stable QDS-composites for solid state lighting and the method of making them through initiator-free polymerization

ABSTRACT

The invention provides a lighting device comprising (i) a light source configured to generate light source light, and (ii) a light converter configured to convert at least part of the light source light into visible converter light, wherein the light converter comprises a polymeric host material with light converter nanoparticles embedded in the polymeric host material, wherein the polymeric host material is based on radical polymerizable monomers, and wherein the polymeric host material contains equal to or less then 5 ppm radical initiator based material relative to the total weight of the polymeric host material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/419,579 filed on Feb. 4, 2015, titled “HIGHLY STABLEQDS-COMPOSITES FOR SOLID STATE LIGHTING AND THE METHOD OF MAKING THEMTHROUGH INITIATOR-FREE POLYMERIZATION”, which is a §371 application ofInternational Application No. PCT/IB2013/055904 filed on Jul. 18, 2013,which claims priority to U.S. Provisional Patent Application No.61/679,956 filed on Aug. 6, 2012. U.S. patent application Ser. No.14/419,579, International Application No. PCT/IB2013/055904, and U.S.Provisional Patent Application No. 61/679,956 are incorporated herein.

FIELD OF THE INVENTION

The invention relates to a lighting device comprising (i) a lightsource, configured to generate light source light, and (ii) a lightconverter, configured to convert at least part of the light source lightinto visible converter light. The invention further relates to a liquidcrystal display device comprising a back lighting unit with suchlighting devices. Further, the invention relates to the light converterper se, as well as to a method for the production of such lightconverter.

BACKGROUND OF THE INVENTION

Quantum dot (QD) based lighting is known in the art. WO2012021643, forinstance, describes systems and methods that relate to quantum dotstructures for lighting applications. In particular, quantum dots andquantum dot containing inks (comprising mixtures of different wavelengthquantum dots) are synthesized for desired optical properties andintegrated with an LED source to create a trichromatic white lightsource. The LED source may be integrated with the quantum dots in avariety of ways, including through the use of a small capillary filledwith quantum dot containing ink or a quantum dot containing film placedappropriately within the optical system. These systems may result inimproved displays characterized by higher color gamuts, lower powerconsumption, and reduced cost. For instance, this document describes amethod of generating trichromatic white light comprising contactinglight from a light source capable of emitting blue light with an opticalmaterial comprising a host material and first quantum dots capable ofemitting green light and second quantum dots capable of emitting redlight, wherein the weight percent ratio of the first quantum dots to thesecond quantum dots in the optical material is in a range from about 9:1to about 2:1, and generating trichromatic white light from a combinationof the light from the light source, the light from the first quantumdots and the light from the second quantum dots.

SUMMARY OF THE INVENTION

Nanoparticles such as quantum dots (QDs) can possess properties whichmake them advanced luminescent materials to be used in solid statelighting. Here below, nanoparticles, such as quantum dots, that have theability to give (visible) luminescence are also indicated as “lightconverter nanoparticles”. They can e.g. be used in converting blue lightto other colors, for obtaining high quality white light with highefficacy. Nanoparticles such as QDs have the advantage of a narrowemission band and color tunablility by varying the size of theparticles. For their applications in solid state lighting it may benecessary to embed the light converter nanoparticles in transmissivesolid matrices such as polymers. One of the ways to obtain such apolymer matrix containing light converter nanoparticles is to first makea mixture of free-radical polymerisable monomers such as one or more ofacrylates and thiolene system with light converter nanoparticles. Inorder to initiate the polymerization of the monomer, an initiator whichcan be activated thermally or photo chemically can be added to themixture. Radical initiators generally possess weak bonds that have smallbond dissociation energies, can produce radical species under mildconditions such as heating and UV irradiation, and initiate free radicalpolymerization of acrylates, vinyls, and thiolene systems.

An example of a thermal initiator is benzoyl peroxide andazo-isobutyro-nitril (AIBN) (see further also below). In addition to oralternative to such azo initiator, a peroxide initiator can also beused. Further, in addition to or alternative to such initiator, alsophoto initiators such as α,α-dimethoxy-α-phenylacetophenone can be used.

While such radicals can initiate polymerization they can also negativelyinfluence the light converter nanoparticles. They can quench theemission of light converter nanoparticles by decreasing both quantumefficiency and stability of the light converter nanoparticles.

Hence, it is an aspect of the invention to provide a lighting deviceand/or light converter (for such lighting device), which preferablyfurther at least partly obviate one or more of above-describeddrawbacks.

Here we suggest using a system which can be polymerized in the absenceof or at extremely low concentration of initiator.

However, it appears that at such low initiator concentrationspolymerization is easily terminated by the presence oxygen. For thispurpose, we further especially suggest removing substantially all theoxygen which may lead to inhibition of polymerization of the monomericmixture.

In such a system, free radicals seem nevertheless to appear to be to beformed by e.g. radiation (optionally while heating), even with radiationhaving a wavelength in the range of 250-470 nm, such as 300-460 nm, suchas at least 300 nm, like 365 nm. For example, light may generate a chainreaction which may lead to the polymerization of the system (in thesubstantial absence of a radical initiator).

We surprisingly found that the systems produced without or an extremelylow amount of the initiator showed three orders of magnitude higherstability than systems produced with the expected amount of radicalinitiator.

We therefore suggest herein the system (further also indicated as “lightconverter”) of initiator free polymerized systems containing lightconverter nanoparticles with high stability as light convertingmaterials for e.g. solid state lighting and the method for producingsuch light converter.

Hence, in a first aspect, the invention provides a lighting device(“device”) comprising:

-   -   a light source configured to generate light source light,    -   a light converter configured to convert at least part of the        light source light into visible converter light, wherein the        light converter comprises a polymeric host material with light        converter nanoparticles embedded in the polymeric host material,        wherein the polymeric host material is based on radical (photo        initiator) polymerizable monomers, and wherein the polymeric        host material contains equal to or less then 5 ppm radical        initiator based material relative to the total weight of the        polymeric host material. Especially, the light converter is        enclosed by an encapsulation, wherein the encapsulation is        configured to reduce exposure of the light converter to O₂. The        combination of light converter and encapsulate is herein also        indicated as light converter unit.

As indicated above such light converter has much better opticalproperties than expected and has optical properties, especially inrespect of stability, that is much larger than in the presence ofsubstantial amounts of radical initiator. Surprisingly, even in theabsence of the radical initiator, the radical polymerizable monomersappear to form a polymer upon irradiation with light and/or due tothermal heating, especially due to irradiation with light, especially UVlight.

Surprisingly, polymerization (substantially) without photo initiator atlong wavelength irradiation such as 365 nm takes place. Thepolymerization reaction may start during UV exposure step, and thencontinue during a heating stage, or during exposure to the emissionlight from the converter nanoparticles, such as blue light. Exposure toblue may cause the temperature in the film to rise, due to <100% quantumyield, meaning that the energy of some of the photons absorbed by thequantum dots is released as heat. This may (further) lead topolymerization. The invention thus allows making systems which havesubstantially no photo initiator, which leads to more stable systemswith more stable optical properties. The luminescence intensity asfunction of time is much more stable for systems according to theinvention, than systems with substantially higher amount of photoinitiator.

Hence, in a further aspect, the invention also provides such lightconverter per se.

The lighting device may be part of or may be applied in e.g. officelighting systems, household application systems, shop lighting systems,home lighting systems, accent lighting systems, spot lighting systems,theater lighting systems, fiber-optics application systems, projectionsystems, self-lit display systems, pixelated display systems, segmenteddisplay systems, warning sign systems, medical lighting applicationsystems, indicator sign systems, decorative lighting systems, portablesystems, automotive applications, green house lighting systems,horticulture lighting, or LCD backlighting

As indicated above, the lighting unit may be used as backlighting unitin an LCD display device. Hence, the invention provides also a LCDdisplay device comprising the lighting unit as defined herein,configured as backlighting unit. The invention also provides in afurther aspect a liquid crystal display device comprising a backlighting unit, wherein the back lighting unit comprises one or morelighting devices as defined herein.

Preferably, the light source is a light source that during operationemits (light source light) at least light at a wavelength selected fromthe range of 200-490 nm, especially a light source that during operationemits at least light at wavelength selected from the range of 400-490nm, even more especially in the range of 440-490 nm. This light maypartially be used by the light converter nanoparticles (see further alsobelow). Hence, in a specific embodiment, the light source is configuredto generate blue light.

In a specific embodiment, the light source comprises a solid state LEDlight source (such as a LED or laser diode).

The term “light source” may also relate to a plurality of light sources,such as 2-20 (solid state) LED light sources. Hence, the term LED mayalso refer to a plurality of LEDs.

The term white light herein, is known to the person skilled in the art.It especially relates to light having a correlated color temperature(CCT) between about 2000 and 20000 K, especially 2700-20000 K, forgeneral lighting especially in the range of about 2700 K and 6500 K, andfor backlighting purposes especially in the range of about 7000 K and20000 K, and especially within about 15 SDCM (standard deviation ofcolor matching) from the BBL (black body locus), especially within about10 SDCM from the BBL, even more especially within about 5 SDCM from theBBL.

In an embodiment, the light source may also provide light source lighthaving a correlated color temperature (CCT) between about 5000 and 20000K, e.g. direct phosphor converted LEDs (blue light emitting diode withthin layer of phosphor for e.g. obtaining of 10000 K). Hence, in aspecific embodiment the light source is configured to provide lightsource light with a correlated color temperature in the range of5000-20000 K, even more especially in the range of 6000-20000 K, such as8000-20000 K. An advantage of the relative high color temperature may bethat there may be a relative high blue component in the light sourcelight.

The terms “violet light” or “violet emission” especially relates tolight having a wavelength in the range of about 380-440 nm. The terms“blue light” or “blue emission” especially relates to light having awavelength in the range of about 440-490 nm (including some violet andcyan hues). The terms “green light” or “green emission” especiallyrelate to light having a wavelength in the range of about 490-560 nm.The terms “yellow light” or “yellow emission” especially relate to lighthaving a wavelength in the range of about 540-570 nm. The terms “orangelight” or “orange emission” especially relate to light having awavelength in the range of about 570-600. The terms “red light” or “redemission” especially relate to light having a wavelength in the range ofabout 600-750 nm. The term “pink light” or “pink emission” refers tolight having a blue and a red component. The terms “visible”, “visiblelight” or “visible emission” refer to light having a wavelength in therange of about 380-750 nm.

The light converter can be seen as a (solid) entity, in generalessentially comprising the polymeric host material and the lightconverter nanoparticles. The latter are in general essentially embeddedin the polymeric host material. Hence, the light converter nanoparticlesare enclosed by the polymeric host material. Especially, the lightconverter nanoparticles are dispersed in the polymeric host material.The polymeric host material is thus especially configured to be apolymer matrix (for the light converter nanoparticles embedded therein).The light converter may be encapsulated (see further below). The lightconverter (and also the light converter unit) may be a film, a layer,such as a self supporting layer, or a body.

The light converter can be configured as light exit window of thelighting device. Hence, in such embodiment, light from the light sourceand converter light (see further below) may emanate from the lightingdevice via and from the light converter (during use of the device). Thelight converter may also be configured in reflective mode. For instance,a light mixing chamber may comprise one or more wall(s) comprising thelight converter (reflective mode) and/or an exit window comprising thelight converter (transmissive mode).

The light converter (or more precisely the light converternanoparticles) is (are) radiationally coupled to the light source (or,as indicated above, a plurality of light sources). The term“radiationally coupled” especially means that the light source and thelight converter are associated with each other so that at least part ofthe radiation emitted by the light source is received by the lightconverter (and at least partly converted into luminescence). The term“luminescence” refers to the emission which emits the light converternanoparticles emit upon excitation by the light source light of thelight source. This luminescence is herein also indicated as converterlight (which at least comprises visible light, see also below).

The light converter will in general also be configured downstream of thelight source. The terms “upstream” and “downstream” relate to anarrangement of items or features relative to the propagation of thelight from a light generating means (here the especially the lightsource), wherein relative to a first position within a beam of lightfrom the light generating means, a second position in the beam of lightcloser to the light generating means is “upstream”, and a third positionwithin the beam of light further away from the light generating means is“downstream”.

The device is especially configured to generate device light, which atleast partly comprises the converter light, but which may optionallyalso comprise (remaining) light source light. For instance, the lightconverter may be configured to only partly convert the light sourcelight. In such instance, the device light may comprise converter lightand light source light. However, in another embodiment the lightconverter may also be configured to convert all the light source light.

Hence, in a specific embodiment, the lighting device is configured toprovide lighting device light comprising both light source light andconverter light, for instance the former being blue light, and thelatter comprising yellow light, or yellow and red light, or green andred light, or green, yellow and red light, etc. In yet another specificembodiment, the lighting device is configured to provide only lightingdevice light comprising only converter light. This may for instancehappen (especially in transmissive mode) when light source lightirradiating the light converter only leaves the downstream side of thelight converter as converted light (i.e. all light source lightpenetrating into the light converter is absorbed by the lightconverter).

The term “light converter” may also relate to a plurality of lightconverters. These can be arranged downstream of one another, but mayalso be arranged adjacent to each other (optionally also even inphysical contact as directly neighboring light converters). Theplurality of light converters may comprise in an embodiment two or moresubsets which have different optical properties. For instance, one ormore subsets may be configured to generate light converter light with afirst spectral light distribution, like green light, and one or moresubsets may be configured to generate light converter light with asecond spectral light distribution, like red light. More than two ormore subsets may be applied. When applying different subsets havingdifferent optical properties, e.g. white light may be provided and/orthe color of the device light (i.e. the converter light and optionalremaining light source light (remaining downstream of the lightconverter). Especially when a plurality of light sources is applied, ofwhich two or more subsets may be individually controlled, which areradiationally coupled with the two or more light converter subsets withdifferent optical properties, the color of the device light may betunable. Other options to make white light are also possible (see alsobelow).

As indicated above, the light converter in general essentially comprisesthe polymeric host material and the light converter nanoparticles.

The phrase “wherein the polymeric host material is based on radicalpolymerizable monomers”, may especially indicate that the polymer hostmaterial is obtainable by reaction monomers that are able to formpolymers by radical polymerization. A non-limiting number of examples ofsuch polymers are mentioned below, and the person skilled in the art mayderive therefrom which monomers (i.e. monomer precursors) may be used(see further also below). Such monomer thus especially includes one ormore radical-polamerizable groups (which may be used for polymerizationwith a photo initiator upon irradiation). Such monomers may in anembodiment include different type of monomers.

As can for instance be derived from WO 03/093328, examples of monomerspolymerizable by a free radical polymerization process include, but arenot limited to, alpha-olefins; dienes such as butadiene and chloroprene;styrene, alpha-methyl styrene, and the like; heteroatom substitutedalpha-olefins, for example, vinyl acetate, vinyl alkyl ethers forexample, ethyl vinyl ether, vinyltrimethylsilane, vinyl chloride,tetrafluoroethylene, chlorotrifluoroethylene, N-(3-dimethylaminopropylmethacrylamide, dimethylaminopropyl methacrylamide, acrylamide,methacrylamide, and similar derivatives; acrylic acids and derivativesfor example, acrylic acid, methacrylic acid, crotonic acid,acrylonitrile, acrylic esters substituted with methoxy, ethoxy, propoxy,butoxy, and similar derivatives for example, methyl acrylate, propylacrylate, butyl acrylate, methyl methacrylate, methyl crotonate,glycidyl methacrylate, alkyl crotonates, and related esters; cyclic andpolycyclic olefin compounds for example, cyclopentene, cyclohexene,cycloheptene, cyclooctene, and cyclic derivatives up to C20; polycyclicderivates for example, norbornene, and similar derivatives up to C20;cyclic vinyl ethers for example, 2, 3-dihydrofuran, 3,4-dihydropyran,and similar derivatives; allylic alcohol derivatives for example,vinylethylene carbonate, disubstituted olefins such as maleic andfumaric compounds for example, maleic anhydride, diethylfumarate, andthe like; and mixtures thereof.

As can be derived from e.g. WO 2011/031871, additional examples ofmonomers include, but are not limited to, allyl methacrylate, benzylmethyl acrylate, 1,3-butanediol dimethacrylate, 1,4-butanedioldimethacrylate, butyl acrylate, n-butyl methacrylate, ethylmethacrylate, 2-ethyl hexyl acrylate, 1,6-hexanediol dimethacrylate,4-hydroxybutyl acrylate, hydroxyethyl acrylate, 2-hydroxyethylmethacrylate, 2-hydroxypropyl acrylate, isobutyl methacrylate, laurylmethacrylate, methacrylic acid, methyl acrylate,2,2,3,3,4,4,5,5-octafluoropentyl acrylate, pentaerythritol triacrylate,2,2,2-trifluoroethyl 2-methyl acrylate, trimethylolpropane triacrylate,acrylamide n,n,-methylene-bisacryl-amide phenyl acrylate, and divinylbenzene.

Many of these types of monomers are acrylate systems. Hence, the term“acrylate” may refer to any of those above mentioned systems such asacrylate, meth(yl)acrylate, butyl acrylate, lauryl methacrylate, etc.etc. Likewise, vinyl monomer may refer to any monomer comprising a vinylgroup.

The phrase “wherein the polymeric host material is based on radicalpolymerizable monomers” does not exclude the presence of e.g.cross-linkers in the monomeric starting material. For the synthesis ofthe light converter, see below.

In principle, the polymer obtained may be any polymer, such as a linearpolymer, a (hyper)branched polymer, a cross-linked polymer, a starpolymer, a dendrimer, a random copolymer, an alternating copolymer, agraft copolymer, a block copolymer, and a terpolymer. The polymeric hostmaterial may in an embodiment be or comprise a resin.

Especially those radical polymerizable monomers are applied, which leadto a light transmissive polymer. In the embodiment of the invention, the(light transmissive polymer) is a polymer which shows high lighttransmission. Preferably a mean absorption of less than 5%/mm morepreferentially less than 2%/mm, especially less than 1%/mm (per mmpolymer thickness) in the wavelength region 400-700 nm. Hence, in anembodiment the first polymer is a polymer having an absorption of lessthan 5%/mm, more preferentially less than 2%/mm and most preferentiallyless than 1%/mm in the wavelength range of 400-700 nm. Note that thetransmission and absorption of the polymer is related to the polymer perse, i.e. the polymeric host material, and not to the transmission of thelight converter (i.e. including the light converter nanoparticles).Especially, the maximum absorption (of the first polymer) is less than20%/mm, even more especially less than 10%/mm, over the entirewavelength region 400-700 nm. Transmission (T) and absorption (A) relateas A=1−To/Ti, wherein Ti the intensity of the visible light impinging onthe item (such as the first polymer or the converter) and To being isthe intensity of the light escaping at the other side of the item. Thetransmission or light permeability can be determined by providing lightat a specific wavelength with a first intensity to the material andrelating the intensity of the light at that wavelength measured aftertransmission through the material, to the first intensity of the lightprovided at that specific wavelength to the material (see also E-208 andE-406 of the CRC Handbook of Chemistry and Physics, 69th edition,1088-1989). Herein, transmissive ma relate to transparent and totranslucent.

As can e.g. be derived from WO 2011/031871, examples of polymers, forexample and without limitation, are polyethylene, polypropylene,polystyrene, polyethylene oxide, polysiloxane, polyphenylene,polythiophene, poly (phenylene-vinylene), polysilane, polyethyleneterephthalate and poly (phenylene-ethynylene), polymethylmethacrylate,polylaurylmethacrylate, polycarbonate, epoxy, and other epoxies. Similaras what has been said with respect to monomers, some of these types ofpolymers are acrylate systems. Hence, the term “polyacrylate” may referto any of those above mentioned systems such as polyacrylate,polymeth(yl)acrylate, polybutyl acrylate, polylauryl methacrylate, etc.etc. Likewise, vinylpolymer may refer to any polymer based on monomerscomprising a vinyl group, such as polyethylene, polyprolylene, etc. etc.

In view of light transmission and/or chemical stability and/orproduction process considerations, especially the polymeric hostmaterial is selected from the group consisting of a poly vinyl polymer(such as a poly ethylene, a poly propylene, etc.), a poly acrylatepolymer (such as a poly acrylate, a poly methacrylate, a polylaurylmethacrylate, etc.) and a thiol-ene polymer (such aspolythiophene).

In some embodiments, though not exclusively, the device is configured insuch a way and the polymeric host material is chosen (designed) in sucha way that during operation of the device, the temperature of the lightconverter is below the glass temperature (Tg) of the polymer of thepolymeric host material, especially at least 10° C. below the glasstemperature. However, other systems wherein above condition is notfulfilled may also be chosen. For instance, polymeric host material mayhave a glass temperature of 130° C. and an application temperature of120° C. To increase the glass temperature, a cross-linker may beincluded in the starting mixture.

Polymers obtained with low concentration of initiator may have highmolecular weight, and therefore have long chains with reduced mobility.This may restrict light converter nanoparticles from migrating in thepolymer film and forming aggregates with red-shifted emission, lowerquantum yield (QY) and fast decaying photo luminescence (PL). Longerpolymer molecules also ensure a higher glass transition temperature ofthe polymer, conferring it stability over a larger temperature interval.

As indicated above, the polymeric host material is substantially freefrom radical initiator based material. This is indicated with the phrase“wherein the polymeric host material contains equal to or less then 5ppm radical initiator based material relative to the total weight of thepolymeric host material.” Hence, the radical initiator content isdefined with respect to the weight of the polymeric host material, andnot with respect to the light converter. Hence, the radical initiatorcontent is the weight percentage of the radical initiator based materialrelative to the total amount of polymer. For instance, starting with 5mg photo initiator, 1 g light converter nanoparticles and 5 kg radicalpolymerizable monomers may lead to a light converter comprising about 1ppm radical initiator based material and about 0.02 wt. % lightconverter nano particles.

In prior art systems, the radical initiator based material content mayfor instance be over 2000 ppm, whereas in the present invention, theamount is 5 ppm or lower, such as ≦1 ppm, even more especially ≦0.1 ppm,yet even more especially ≦0.01 ppm, like at maximum 1 ppb. Especially,the polymeric host material contains equal to or less then 5 ppm, butmore than 0 ppm, such as at least 0.1 ppb, like at least 0.01 ppm,radical initiator based material relative to the total weight of thepolymeric host material. Especially, when starting with such amounts ofradical initiator (photo initiator), good results may be obtained interms of polymerization (with relative high wavelength radiation) andstability of the nanoparticles (QDs) and/or light converter.

The term “radical initiator based material” refers to the remains of theradical initiator that can be found or evaluation from the compositionof the polymeric host material. This radical initiator based materialmay include unreacted radical initiator, but also radical initiator thathas been reacted. In case radical initiator has been consumed, it refersto groups in the polymeric host material that originate from the radicalinitiator. Assuming that the starting materials for the polymeric hostmaterial only comprises radical polymerizable monomers, and relative tothe total amount of radical polymerizable monomers 1 ppm radicalinitiator is added, the amount of radical initiator based material inthe polymeric host material will also be 1 ppm. The term “radicalinitiator” may in an embodiment refer to a plurality of differentradical initiators.

The free radical polymerization process is well known and involves areaction initiated by the formation of a free radical from a freeradical generator, for example a peroxide or azo initiator. A reactionis initiated by addition of the free radical to an unsaturated monomermolecule that subsequently adds, in a step-wise manner, to additionalunsaturated monomers to form a growing chain or polymer.

As can e.g. be derived from WO 03/093328, examples of free radicalinitiators include, but are not limited to, the following: organicperoxides like: t-alkyl peroxyesters, tert-butyl peroxybenzoate,tert-butyl peroxyacetate, ter-butyl peroxypivalate, tert-butylperoxymaleate, monoperoxycarbonates, OO-tert-butyl O-isopropylmonoperoxycarbonate, diperoxyketals, ethyl 3,3-di-(tert-amylperoxy)-butyrate, n-butyl-4,4-di(tertbutylperoxy)-valerate, 1,1-di (tert-butylperoxy)-cyclohexane,1,1-di (tert-amylperoxy)-cyclohexane, dialkyl peroxides, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, di-tert-amyl peroxide,di-tert-butyl peroxide, dicumyl peroxide, t-alkyl hydroperoxides,tert-butyl hydroperoxide, ter-amyl hydroperoxide, alpha-cumylhydroperoxide, ketone peroxides, methyl ethyl ketone peroxide,cyclohexanone peroxide, 2,4-pentanedione peroxide, isobutyryl peroxide,isopropyl peroxydicarbonate, di-n-butyl peroxydicarbonate, di-sec-butylperoxydicarbonate, tert-butyl perneodecanoate, dioctanoyl peroxide,didecanoyl peroxide, diproprionyl peroxide, didecanoyl peroxide,dipropionyl peroxide, dilauroyl peroxide, tert-butyl perisobutyrate,tert-butyl peracetate, tert-butyl per-,5, 5-trimethyl hexanoate; azocompounds like: 2,2′-azobis [4-methoxy-2,4-dimethyl]pentanenitrile,2,3′-azobis [2,4-dimethyl] pentanenitrile, 2,2′-azobis[isobutyronitrile]; carbon-carbon initiators like:2,3-dimethyl-2,3-diphenylbutane, 3,4-dimethyl-3,4-diphenylhexane,1,1,2,2-tetraphenyl-1,2-bis (trimethylsiloxy) ethane; inorganicperoxides like: hydrogen peroxide, potassium peroxydisulfate;photoinitiators like: benzophenone 4-phenylbenzophenone, xanthonethioxanthone, 2-chlorothioxanthone, 4,4′-bis (N,N′-dimethylaminobenzophenone), benzyl, 9,10-phenanthraquinone, 9,10-anthraquinone,alpha,alpha-dimethyl-alpha-hydroxyacetophenone,(1-hydroxycyclohexyl)-phenylmethanone, benzoin ethers, like methyl,ethyl, isobutyl, benzoin ethers,alpha,alpha-dimethoxy-alpha-phenylacetophenone,1-phenyl-1,2-propanedione, 2-(O-benzoyl)oxime,diphenyl(2,4,6-trimethylbenzoyl)phosphineoxide,alpha-dimethylamino-alpha-ethyl-alpha-benzyl-3,5-dimethyl-4-morpholinoacetophenone,etc.

As can for instance be derived from WO 2011/031871, there are in generaltwo classes of photoinitiators. In the first class, the chemicalundergoes unimolecular bond cleavage to yield free radicals. Examples ofsuch photoinitiators include benzoin ethers, benzyl ketals,a-dialkoxy-acetophenones, a-amino-alkylphenones, and acylphosphineoxides. The second class of photoinitiators is characterized by abimolecular reaction where the photo initiator reacts with aco-initiator to form free radicals. Examples of such arebenzophenones/amines, thioxanthones/amines, and titanocenes (visiblelight). A non-exhaustive listing of specific examples of photoinitiatorsthat may be useful with a photo-polymerizable monomer for particlepreparation include the following from CIBA: IRGACURE 184(1-hydroxy-cyclohexyl-phenyl-ketone), DAROCUR 1173(2-hydroxy-2-methyl-1-phenyl-1-propanone), IRGACURE 2959(2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone), DAROCURMBF (Methylbenzoylformate), IRGACURE 754 (oxy-phenyl-acetic acid 2-[2oxo-2 phenyl-acetoxy-ethoxy]-ethyl ester and oxy-phenyl-acetic2-[2-hydroxy-ethoxy]-ethyl ester), IRGACURE 651 Alpha,(alpha-dimethoxy-alpha-phenylacetophenone), IRGACURE 369(2-Benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone),IRGACURE 907(2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone),DAROCUR TPO (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide), IRGACURE819 (phosphine oxide, phenyl bis (BAPO) (2,4,6-trimethyl benzoyl)),IRGACURE 784 (bis(eta5-2,4-cyclopentadien-1-yl)Bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium),IRGACURE 250(iodonium,(4-methylphenyl)[4-(2-methylpropyl)phenyl]-hexafluorophosphate(1-).)As indicated above, the total amount of radical initiator, of whichexamples are given above, relative to the polymeric host material isequal to or less then 5 ppm, like at maximum 1 ppm, such as at maximumeven only 1 ppb.

The quantum dots or luminescent nanoparticles, which are hereinindicated as light converter nanoparticles, may for instance comprisegroup II-VI compound semiconductor quantum dots selected from the groupconsisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS,CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS,CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS,CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe andHgZnSTe. In another embodiment, the luminescent nanoparticles may forinstance be group III-V compound semiconductor quantum dots selectedfrom the group consisting of GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP,InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs,GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, andInAlPAs. In yet a further embodiment, the luminescent nanoparticles mayfor instance be I-III-VI2 chalcopyrite-type semiconductor quantum dotsselected from the group consisting of CuInS₂, CuInSe₂, CuGaS₂, CuGaSe₂,AgInS₂, AgInSe₂, AgGaS₂, and AgGaSe₂. In yet a further embodiment, theluminescent nanoparticles may for instance be I-V-VI2 semiconductorquantum dots, such as selected from the group consisting of LiAsSe₂,NaAsSe₂ and KAsSe₂. In yet a further embodiment, the luminescentnanoparticles may for instance be a group IV-VI compound semiconductornano crystals such as SbTe. In a specific embodiment, the luminescentnanoparticles are selected from the group consisting of InP, CuInS₂,CuInSe₂, CdTe, CdSe, CdSeTe, AgInS₂ and AgInSe₂. In yet a furtherembodiment, the luminescent nanoparticles may for instance be one of thegroup II-VI, III-V, I-III-V and IV-VI compound semiconductor nanocrystals selected from the materials described above with inside dopantssuch as ZnSe:Mn, ZnS:Mn. The dopant elements could be selected from Mn,Ag, Zn, Eu, S, P, Cu, Ce, Tb, Au, Pb, Tb, Sb, Sn and Tl. Herein, theluminescent nanoparticles based luminescent material may also comprisedifferent types of QDs, such as CdSe and ZnSe:Mn.

It appears to be especially advantageous to use II-VI quantum dots.Hence, in an embodiment the semiconductor based luminescent quantum dotscomprise II-VI quantum dots, especially selected from the groupconsisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS,CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS,CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS,CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe andHgZnSTe, even more especially selected from the group consisting of CdS,CdSe, CdSe/CdS and CdSe/CdS/ZnS.

The luminescent nanoparticles (without coating) may have dimensions inthe range of about 2-50 nm, such as 2-20 nm, especially 2-10 nm, evenmore especially 2-5 nm; especially at least 90% of the nanoparticleshave dimension in the indicated ranges, respectively, (i.e. e.g. atleast 90% of the nanoparticles have dimensions in the range of 2-50 nm,or especially at least 90% of the nanoparticles have dimensions in therange of 2-5 nm). The term “dimensions” especially relate to one or moreof length, width, and diameter, dependent upon the shape of thenanoparticle.

In an embodiment, the light converter nanoparticles have an averageparticle size in a range from about 1 to about 1000 nanometers (nm), andpreferably in a range from about 1 to about 100 nm. In an embodiment,nanoparticles have an average particle size in a range from about 1 toabout 20 nm. In an embodiment, nanoparticles have an average particlesize in a range from about 1 to about 10 nm.

Typical dots are made of binary alloys such as cadmium selenide, cadmiumsulfide, indium arsenide, and indium phosphide. However, dots may alsobe made from ternary alloys such as cadmium selenide sulfide. Thesequantum dots can contain as few as 100 to 100,000 atoms within thequantum dot volume, with a diameter of 10 to 50 atoms. This correspondsto about 2 to 10 nanometers. For instance, spherical particles such asCdSe, InP, or CuInSe₂, with a diameter of about 3 nm may be provided.The luminescent nanoparticles (without coating) may have the shape ofspherical, cube, rods, wires, disk, multi-pods, etc., with the size inone dimension of less than 10 nm. For instance, nanorods of CdSe withthe length of 20 nm and a diameter of 4 nm may be provided. Hence, in anembodiment the semiconductor based luminescent quantum dots comprisecore-shell quantum dots. In yet another embodiment, the semiconductorbased luminescent quantum dots comprise dots-in-rods nanoparticles. Acombination of different types of particles may also be applied. Here,the term “different types” may relate to different geometries as well asto different types of semiconductor luminescent material. Hence, acombination of two or more of (the above indicated) quantum dots orluminescent nano-particles may also be applied.

One example, such as derived from WO 2011/031871, of a method ofmanufacturing a semiconductor nanocrystal is a colloidal growth process.Colloidal growth occurs by injection an M donor and an X donor into ahot coordinating solvent. One example of a preferred method forpreparing monodisperse semiconductor nanocrystals comprises pyrolysis oforganometallic reagents, such as dimethyl cadmium, injected into a hot,coordinating solvent. This permits discrete nucleation and results inthe controlled growth of macroscopic quantities of semiconductornanocrystals. The injection produces a nucleus that can be grown in acontrolled manner to form a semiconductor nanocrystal. The reactionmixture can be gently heated to grow and anneal the semiconductornanocrystal. Both the average size and the size distribution of thesemiconductor nanocrystals in a sample are dependent on the growthtemperature. The growth temperature necessary to maintain steady growthincreases with increasing average crystal size. The semiconductornanocrystal is a member of a population of semiconductor nanocrystals.As a result of the discrete nucleation and controlled growth, thepopulation of semiconductor nanocrystals that can be obtained has anarrow, monodisperse distribution of diameters. The monodispersedistribution of diameters can also be referred to as a size. Preferably,a monodisperse population of particles includes a population ofparticles wherein at least about 60% of the particles in the populationfall within a specified particle size range. A population ofmonodisperse particles preferably deviate less than 15% rms(root-mean-square) in diameter and more preferably less than 10% rms andmost preferably less than 5%.

In an embodiment, nanoparticles can comprise semiconductor nanocrystalsincluding a core comprising a first semiconductor material and a shellcomprising a second semiconductor material, wherein the shell isdisposed over at least a portion of a surface of the core. Asemiconductor nanocrystal including a core and shell is also referred toas a “core/shell” semiconductor nanocrystal.

For example, the semiconductor nanocrystal can include a core having theformula MX, where M can be cadmium, zinc, magnesium, mercury, aluminum,gallium, indium, thallium, or mixtures thereof, and X can be oxygen,sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony, ormixtures thereof. Examples of materials suitable for use assemiconductor nanocrystal cores include, but are not limited to, ZnO,ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe,GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AIN, AlP, AlSb,TIN, TIP, TlAs, TlSb, PbO, PbS, PbSe, PbTe, Ge, Si, an alloy includingany of the foregoing, and/or a mixture including any of the foregoing,including ternary and quaternary mixtures or alloys.

The shell can be a semiconductor material having a composition that isthe same as or different from the composition of the core. The shellcomprises an overcoat of a semiconductor material on a surface of thecore semiconductor nanocrystal can include a Group IV element, a GroupII-VI compound, a Group II-V compound, a Group III-VI compound, a GroupIII-V compound, a Group IV-VI compound, a Group I-III-VI compound, aGroup II-IV-VI compound, a Group II-IV-V compound, alloys including anyof the foregoing, and/or mixtures including any of the foregoing,including ternary and quaternary mixtures or alloys. Examples include,but are not limited to, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS,MgSe, GaAs, GaN, GaP, GaSe, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP,InSb, AlAs, AIN, AlP, AlSb, TIN, TIP, TlAs, TlSb, PbO, PbS, PbSe, PbTe,Ge, Si, an alloy including any of the foregoing, and/or a mixtureincluding any of the foregoing. For example, ZnS, ZnSe or CdSovercoatings can be grown on CdSe or CdTe semiconductor nanocrystals. Anovercoating process is described, for example, in U.S. Pat. No.6,322,901. By adjusting the temperature of the reaction mixture duringovercoating and monitoring the absorption spectrum of the core, overcoated materials having high emission quantum efficiencies and narrowsize distributions can be obtained. The overcoating may comprise one ormore layers. The overcoating comprises at least one semiconductormaterial which is the same as or different from the composition of thecore. Preferably, the overcoating has a thickness from about one toabout ten monolayers. An overcoating can also have a thickness greaterthan ten monolayers. In an embodiment, more than one overcoating can beincluded on a core.

In an embodiment, the surrounding “shell” material can have a band gapgreater than the band gap of the core material. In certain otherembodiments, the surrounding shell material can have a band gap lessthan the band gap of the core material.

In an embodiment, the shell can be chosen so as to have an atomicspacing close to that of the “core” substrate. In certain otherembodiments, the shell and core materials can have the same crystalstructure.

Examples of semiconductor nanocrystal (core)shell materials include,without limitation: red (e.g., (CdSe)ZnS (core)shell), green (e.g.,(CdZnSe)CdZnS (core)shell, etc.), and blue (e.g., (CdS)CdZnS (core)shell(see further also above for examples of specific light converternanoparticles, based on semiconductors.

Especially, the light converter comprises 0.001-25 wt. % light converternanoparticles relative to the total weight of the light converter, suchas 0.1-20 wt. %, especially not more than 5 wt. %.

In an embodiment, semiconductor nanocrystals preferably have ligandsattached thereto, such as e.g. described in WO 2011/031871. In anembodiment, the ligands can be derived from the coordinating solventused during the growth process. In an embodiment, the surface can bemodified by repeated exposure to an excess of a competing coordinatinggroup to form an overlayer.

For example, a dispersion of the capped semiconductor nanocrystal can betreated with a coordinating organic compound, such as pyridine, toproduce crystallites which disperse readily in pyridine, methanol, andaromatics but no longer disperse in aliphatic solvents. Such a surfaceexchange process can be carried out with any compound capable ofcoordinating to or bonding with the outer surface of the semiconductornanocrystal, including, for example, phosphines, thiols, amines andphosphates. The semiconductor nanocrystal can be exposed to short chainpolymers which exhibit an affinity for the surface and which terminatein a moiety having an affinity for a liquid medium in which thesemiconductor nanocrystal is suspended or dispersed. Such affinityimproves the stability of the suspension and discourages flocculation ofthe semiconductor nanocrystal.

More specifically, the coordinating ligand can have the formula:(Y-)k-n-(X)-(-L)n

wherein k is 2, 3 4, or 5, and n is 1, 2, 3, 4 or 5 such that k-n is notless than zero; X is O, OS, O—Se, O—N, O—P, O—As, S, S=0, S02, Se, Se=0,N, N=0, P, P=0, C=0 As, or As=0; each of Y and L, independently, is H,OH, aryl, heteroaryl, or a straight or branched C2-18 hydrocarbon chainoptionally containing at least one double bond, at least one triplebond, or at least one double bond and one triple bond. The hydrocarbonchain can be optionally substituted with one or more C1-4 alkyl, C2-4alkenyl, C2-4 alkynyl, C1-4 alkoxy, hydroxyl, halo, amino, nitro, cyano,C3-5 cycloalkyl, 3-5 membered heterocycloalkyl, aryl, heteroaryl, C1-4alkylcarbonyloxy, C1-4 alkyloxycarbonyl, C1-4 alkylcarbonyl, or formyl.The hydrocarbon chain can also be optionally interrupted by —O—, —S—,—N(Ra)—, —N(Ra)—C(O)—O—, —O—C(O)—N(Ra)—, —N(Ra)—C(O)—N(Rb)—, —O—C(O)O—,—P(Ra)—, or —P(O)(Ra)—. Each of Ra and Rb, independently, is hydrogen,alkyl, alkenyl, alkynyl, alkoxy, hydroxylalkyl, hydroxyl, or haloalkyl.An aryl group is a substituted or unsubstituted cyclic aromatic group.Examples include phenyl, benzyl, naphthyl, tolyl, anthracyl,nitrophenyl, or halophenyl. A heteroaryl group is an aryl group with oneor more heteroatoms in the ring, for instance furyl, pyridyl, pyrrolyl,phenanthryl.

A suitable coordinating ligand can be purchased commercially or preparedby ordinary synthetic organic techniques, for example, as described inJ. March, Advanced Organic Chemistry. Other ligands are described inU.S. patent application Ser. No. 10/641,292 for “StabilizedSemiconductor Nanocrystals”, filed 15 Aug. 2003, which issued on 9 Jan.2007 as U.S. Pat. No. 7,160,613, which is hereby incorporated byreference in its entirety. Other examples of ligands includebenzylphosphonic acid, benzylphosphonic acid including at least onesubstituent group on the ring of the benzyl group, a conjugate base ofsuch acids, and mixtures including one or more of the foregoing. In anembodiment, a ligand comprises 4-hydroxybenzylphosphonic acid, aconjugate base of the acid, or a mixture of the foregoing. In anembodiment, a ligand comprises3,5-di-tert-butyl-4-hydroxybenzylphosphonic acid, a conjugate base ofthe acid, or a mixture of the foregoing. Additional examples of ligandsthat may be useful with the present invention are described inInternational Application No. PCT/US2008/010651, filed 12 Sep. 2008, ofBreen, et al., for “Functionalized Nanoparticles And Method” andInternational Application No. PCT/US2009/004345, filed 28 Jul. 2009 ofBreen et al., for “Nanoparticle Including Multi-Functional Ligand AndMethod”, each of the foregoing being hereby incorporated herein byreference.

In an embodiment, the light converter can optionally further be providedwith a cover, coating or layer for protection from the environment(e.g., dust, moisture, and the like) and/or scratching or abrasion.

Especially, the light converter is enclosed by an encapsulate orencapsulation. As used herein, “encapsulation” may refer to protectionagainst a particular element or compound, for example, oxygen (O₂)(suchas in the form of air) and/or water. In an embodiment, encapsulation canbe complete (also referred to herein as full encapsulation).

In an embodiment, encapsulation can be less than complete (also referredto herein as partial encapsulation). Hence, in an embodiment, theoptical material is at least partially encapsulated. Therefore, in anembodiment the optical material is at least partially encapsulated by abarrier material. Especially, in an embodiment the optical material isat least partially encapsulated by a material that is substantiallyimpervious to oxygen. In an embodiment, the optical material is at leastpartially encapsulated by a material that is substantially impervious tomoisture (e.g. water). In an embodiment, the optical material is atleast partially encapsulated by a material that is substantiallyimpervious to air. In an embodiment, the optical material is at leastpartially encapsulated by a material that is substantially impervious tooxygen and moisture.

In an embodiment, for example, the optical material can be sandwichedbetween substrates. In an embodiment, one or both of the substrates cancomprise glass plates. In an embodiment, for example, the opticalmaterial can be sandwiched between a substrate (e.g., a glass plate) anda barrier film. In an embodiment, the optical material can be sandwichedbetween two barrier films or coatings.

In an embodiment, the optical material is fully encapsulated. In anembodiment, for example, the optical material can be sandwiched betweensubstrates (e.g., glass plates) that are encapsulated by a perimeterencapsulation. In an embodiment, for example, the optical material canbe disposed on a substrate (e.g., a glass support) and fully covered bybarrier film. In an embodiment, for example, the optical material can bedisposed on a substrate (e.g., a glass support) and fully covered byprotective coating. In an embodiment, the optical material can besandwiched between two barrier films or coatings that are encapsulatedby a perimeter encapsulation. Examples of suitable barrier films orcoatings include, without limitation, a hard metal oxide coating, a thinglass layer, and Barix coating materials available from Vitex Systems,Inc. Other barrier films or coatings can be readily ascertained by aperson skilled in the art.

In an embodiment, more than one barrier film or coating can be used toencapsulate the optical material. Hence, also a multi-layer may beapplied to form the barrier film or coating, or a multi-layer of barrierfilms or coatings may be applied to provide the encapsulation

Especially, the light converter is enclosed by an encapsulation,especially an oxygen non-permeable encapsulation. Hence, theencapsulation is especially configured to block transport of oxygen fromthe atmosphere into the encapsulated light converter. The encapsulationmay comprise different parts. For instance, the encapsulation maycomprise two transmissive plates, between which the light converter issandwiched, and a coating, film, or glue for final enclosing the edgesof the light converter. The light converter can also be prepared in acontainer comprising a light transmissive material, which afterpreparation is closed with a (transmissive) cover, which may be glued,or welded to the container (edges). Hence, any solid material in whichthe light converter can be contained and which especially can blockoxygen transport can be considered an encapsulation. In a specificembodiment, the encapsulation comprises a container containing the lightconverter, and a cover, wherein the container and cover are attached toeach other and enclose the light converter. At least part of theencapsulation is transmissive for light, especially in the visible, andwill thereby allow excitation light reach the light converter nanoparticles and allow emission light therefrom (at least in the visible)escape from the encapsulated light converter.

As will be clear to a person skilled in the art, part of theencapsulation is transmissive for light, especially transmissive forlight of the light source and for light generated by the light converterwhen being irradiated with the light source light (see also above).Especially, the encapsulation should preferably have an oxygenpermeability of at most 10E-04 mL/m2/24 hours, at 10° C. and 85%relative humidity, especially at most 10E-05 mL/m2/24 hours, at 10° C.and 85% relative humidity. Water permeability should preferably be atmost 10E-05 g/m2/24 hours at 10° C., especially at most 10E-06 g/m2/24hours at 10° C. Hence, the light converter unit may especially include aO₂ and H₂O non-permeable encapsulation, which encapsulation especiallyencloses a substantial part of the light converter, even more especiallythe entire light converter. Hence, the encapsulation may enclose thelight converter over its entire perimeter.

In yet a further aspect, the invention provides a method for theproduction of a light converter, the light converter comprising apolymeric host material with light converter nanoparticles embedded inthe polymeric host material, the method comprising:

-   -   providing a mixture (“starting mixture” or “starting materials”)        comprising radical polymerizable monomers, light converter        nanoparticles, and optionally radical initiator;    -   polymerizing the radical polymerizable monomers under low oxygen        conditions, preferably under an inert atmosphere, thereby        providing the polymeric host material with light converter        nanoparticles embedded in the polymeric host material;

wherein equal to or less then 5 ppm radical initiator relative to thetotal weight of the polymeric host material is applied.

In this way, the light converter as further defined above may beobtained. Especially, the method further includes enclosing the thusobtained light converter by an encapsulation (see also above).

Also other species (in addition to the monomers and the light converternanoparticles) may be present in the starting mixture and may beincorporated in the polymeric host material. For instance, reflectiveparticles like TiO₂ particles may also be incorporated. Also inorganicluminescent material, not having nanoparticle character, like micronsized particulate inorganic luminescent materials may be present, aswell as the above indicated cross-linker. Information about the monomersand the light converter nanoparticles, as well as about the optionalradical initiator, are indicated above. As can also be derived from theabove, the mixture may comprise 0.001-25 wt. % light converternanoparticles relative to the total weight of the mixture.

Especially, the radical polymerizable monomers are selected from thegroup consisting of a vinyl monomer, an acrylate monomer, and acombination of a thiol and a diene.

The amount of radical initiator—if any—is here related to the weight ofthe polymeric host material. When a radical initiator is applied,especially a photo initiator is applied. Especially, the startingmixture contains equal to or less then 5 ppm, but more than 0 ppm, suchas at least 0.1 ppb, like at least 0.01 ppm, radical initiator relativeto the total weight of the polymeric host material (that is producedwith the method). Especially, when starting with such amounts of radicalinitiator (photo initiator), good results may be obtained in terms ofpolymerization (with relative high wavelength radiation) and stabilityof the nanoparticles (QDs) and/or light converter.

The polymerization may be started by heating and or irradiating theradical polymerizable polymers, especially may be started by (at least)irradiating the radical polymerizable monomers. Especially,polymerization may be initiated photo chemically upon irradiation withhigh energetic rays such as UV, X-rays, gamma rays, electrons. If in thesubstantial absence of radical (photo)initiator the polymerization maybe started by (e.g. UV) irradiation of the mixture (including theradical polymerizable monomers). In some cases it may be desirable toheat up the mixture above the glass transition of the system in order toreach complete polymerization. When polymerization starts, thetemperature may again be lowered below the glass transition temperature;after termination, the thus obtained light converter may in someembodiments be cooled down below the glass transition temperature.However, also other methods may be applied, as will be clear to theperson skilled in the art. Especially, during polymerization thetemperature will not be higher than the boiling point of the monomer(s)used.

Even with radiation having a wavelength in the range of 250-470 nm, suchas 300-460 nm, such as even at least 300 nm, like even above 350 nm,such as 365 nm, polymerization of the system (in the substantial absenceof a radical initiator) may be photo initiated. An advantage of theselarge wavelength photo initiation, such as at wavelengths larger than300 nm, especially larger than 350 nm, may be that the penetration depthof the light can be larger, which may lead to better and/or morehomogeneous polymerization (for even thicker polymeric layers).

Preferably, before polymerization starts (substantially), the partialpressure of oxygen over the mixture is substantially reduced. Forinstance, the mixture may be provided in a low-oxygen atmosphere, orafter providing the mixture but before polymerization, the oxygenpartial pressure is lowered. In an embodiment, the polymerization takesplace in a low-oxygen environment, like a glove box. Especially, aninert gas may be applied, like one or more of Ar, CO₂ or N₂. Optionally,polymerization may take place under reduced pressure. Alternatively, theoxygen amount in the gas over the mixture, at least duringpolymerization, is less than 1 ppm, such as less than 0.2 ppm. Hence,the method may especially comprise polymerizing the radicalpolymerizable monomers while maintaining the mixture in an inert gasatmosphere.

Further, the method may comprise enclosing the thus obtained lightconverter by an encapsulation, especially an oxygen non-permeableencapsulation. Especially, this encapsulation is applied while the lightconverter is still under the reduced oxygen atmosphere. Examples ofencapsulation are given above.

The term “substantially” herein, such as in “substantially all light” orin “substantially consists”, will be understood by the person skilled inthe art. The term “substantially” may also include embodiments with“entirely”, “completely”, “all”, etc. Hence, in embodiments theadjective substantially may also be removed. Where applicable, the term“substantially” may also relate to 90% or higher, such as 95% or higher,especially 99% or higher, even more especially 99.5% or higher,including 100%. The term “comprise” includes also embodiments whereinthe term “comprises” means “consists of”.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

The devices herein are amongst others described during operation. Aswill be clear to the person skilled in the art, the invention is notlimited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention further applies to a device comprising one or more of thecharacterizing features described in the description and/or shown in theattached drawings. The invention further pertains to a method or processcomprising one or more of the characterising features described in thedescription and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order toprovide additional advantages. Furthermore, some of the features canform the basis for one or more divisional applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIGS. 1a-1c schematically depict some aspects of the device(s) of theinvention;

FIG. 2 schematically depicts an embodiment of the method of theinvention.

The drawings are not necessarily on scale.

FIGS. 3-5 show some experimental results.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1a schematically depicts a lighting device 1 comprising a lightsource 10 configured to generate light source light 11 and a lightconverter 100 configured to convert at least part of the light sourcelight 11 into visible converter light 121. Here schematically only onelight source 10 is depicted. However, more than one light source 10 maybe present.

The light converter has an upstream side 101, which is at least partlydirected to the light source 10, and a downstream side, which faces awayfrom the light source 10 (in this transmissive configuration).

The light converter 100 comprises a polymeric host material 110 withlight converter nanoparticles 120 embedded in the polymeric hostmaterial 110. These can be dots, rods, a combination thereof, etc. (seealso above). The light converter nanoparticles 120 generate uponexcitation by the light source light 11 visible converter light (andoptionally also non-visible radiation, like IR radiation). At least partform the converter light 121 escapes from the downstream side 102 aslighting device light 5. This lighting device light 5, of which at leastpart is in the visible, at least contains part of the convert light 121,and may optionally also contain some remaining light source light 11.

FIG. 1a schematically depicts the lighting device in operation.

FIG. 1b schematically depicts another embodiment, wherein the lightconverter 100 is encapsulated. An encapsulation 400 encloses the lightconverter; this encapsulation may substantially block oxygen (and/orH₂O) transporter from the atmosphere to the light converter. This mayadd to the stability of the light converter nanoparticles 120 (and thepolymeric host). The combination of light converter 100 andencapsulation 400 is herein also indicated as light converter unit 1100.

FIG. 1c schematically depicts one of the applications of the lightingunit 1, here in a liquid crystal display device 2, which comprises aback lighting unit 200 which comprises one or more lighting units 1(here, one lighting unit is schematically depicted), as well as a LCDpanel 300, which can be backlighted with the lighting device light 5 ofthe lighting unit(s) 100 of the back lighting unit 200.

FIG. 2 schematically depicts an embodiment of the method of theinvention, which may comprise providing a mixture comprising radicalpolymerizable monomers 109 and light converter nanoparticles 120 (andoptionally radical initiator; not depicted). The mixture may be providedin a container 401, having walls 404. Part of these walls may betransmissive for light source light and/or converter light (see alsoFIG. 1b ).

Preferably, after providing, or even already during this mixture, theoxygen amount in the atmosphere over the mixture is kept low, forinstance by reducing the partial pressure. This may be done byevacuation and/or introduction of an inert gas. This is indicated withthe symbol “—O₂”. Further, also especially water vapor may be removed(at the same time and/or in the same way; this is indicated with thesymbol “—H₂O”).

Thereafter, polymerization may take place, i.e. polymerizing the radicalpolymerizable monomers 109 under low oxygen conditions, preferably underan inert atmosphere, thereby providing the polymeric host material 110with light converter nanoparticles 120 embedded in the polymeric hostmaterial 110. Especially, polymerization is started by irradiating theradical polymerizable monomers (indicated by the symbol “+hv”).

After polymerization, the thus obtained light converter 100 may beentirely encapsulated, such as with a closure 402 configured to close acontainer opening 405. Optionally, such closure may be welded or gluedor otherwise be connected to the container walls 404 in a sealingconnection. In this way, the light converter unit 1100 is obtained (seealso FIG. 1b ), which comprises the thus obtained light converter 100enclosed by an encapsulation, especially an oxygen non-permeableencapsulation 402,404. One or more parts of this encapsulation 402,404may be transmissive for light source light and/or converter light (seealso FIG. 1b ).

Hence, the invention provides a polymer composite obtained by freeradical polymerization comprising luminescent light converternanoparticles containing low concentration of photo initiator.Especially, the concentration of the initiator is less than 1 ppm andmore preferentially less than 1 ppb. In fact, a photo curable monomermay be applied without photo initiator. Especially, polymerization takesplace under low oxygen conditions, preferably under an inert atmosphere,and with low or zero radical initiator content.

In an embodiment, the polymer is a poly vinyl, polyacrylate or athiolene system. In an embodiment, the composite comprises across-linked network; the cross links are chemical cross links.Especially, the composite may be sealed from atmosphere. Thisencapsulation may be a hermetic encapsulation. In an embodiment, theencapsulation is thin film packaging.

The quantum dots may have the quantum yield of 60% or above at roomtemperature. The concentration of light converter nanoparticles ispreferentially less than 20%. Optionally, the quantum dots compriseligands which can copolymerize with the polymer.

The invention further provides a method of producing the compositeinvolving making a monomeric mixture comprising light converternanoparticles removing oxygen from the system and then placing themixture in a confinement and initiating polymerization photo chemicallyupon irradiation with high energetic rays such as UV, X-rays, gammarays, electrons.

Optionally, the composite may be used in combination with one or morelight converting phosphors for producing white light for illumination.The composite can be used in lighting device for backlighting for LCD.

The light converter nanoparticle emission may especially be at least inthe red part of the visible spectrum (especially peak position between610-620 nm).

EXPERIMENTAL

A plurality of systems was made, under different conditions. Here below,one example is given.

A mixture containing 5 wt. % QDs (CdSe with ZnS shell) and acrylatemonomer were produced (with various amounts of photo radical initiator).The mixture was then placed in an environment with low O₂ and H₂Oconcentration<5 ppm to remove oxygen.

Subsequently the mixture was placed between glass plates and exposed toUV radiation (>=1 w/cm²) 365 nm for initiation of polymerization toobtain a solid polymer containing luminescent QDs. Samples were producedin the presence and absence of photo initiator.

The samples were then tested by irradiating them with laser lightemitting at 450 nm (0.4 W/cm²) at 100° C. and measuring the intensity ofthe luminescence from quantum dots in Nitrogen atmosphere. The resultsare shown in FIGS. 3-5, with on the y-axis relative intensity inarbitrary units).

The sample with 1 wt. % initiator (FIG. 3) had a substantial lowerstability than the sample with 0.1 wt. % photo initiator (FIG. 4); whichwas also not very stable. The sample without photo initiator was verystable (FIG. 5).

Photoluminescence stability measurements were performed for at least 500hours (continuous irradiation with the 450 nm light).

The invention claimed is:
 1. A device comprising: a light source forgenerating light source light, a light converter for converting at leastpart of the light source light into visible converter light, the lightconverter comprising: a polymeric host material; light converternanoparticles embedded in the polymeric host material; wherein: thepolymeric host material is based on radical polymerizable monomers, andthe polymeric host material contains equal to or less than 5 ppm radicalinitiator based material relative to the total weight of the polymerichost material; and first and second substrates, wherein the first andsecond substrates sandwich the polymeric host material and the lightconverter nanoparticles.
 2. The device of claim 1 wherein the first andsecond substrates are glass plates.
 3. The device of claim 1 wherein thefirst and second substrates are barrier films.
 4. The device of claim 1wherein the first substrate is a glass plate and the second substrate isa barrier film.
 5. The device of claim 1 wherein the first substrate isa hard metal oxide coating.
 6. The device of claim 1 wherein the firstand second substrates are encapsulated by a perimeter encapsulation. 7.The device of claim 1 wherein the first substrate is a multi-layer film.8. The device of claim 1 wherein the first and second substrates areoxygen non-permeable.
 9. The device of claim 1 further comprising gluedisposed on edges of the light converter.
 10. The device of claim 1wherein the first substrate comprises a container in which the polymerichost material and the light converter nanoparticles are disposed, andthe second substrate comprises a cover glued or welded to edges of thecontainer.
 11. A device comprising: a light source for generating lightsource light, a light converter for converting at least part of thelight source light into visible converter light, the light convertercomprising: a polymeric host material; and light converter nanoparticlesembedded in the polymeric host material; wherein: the polymeric hostmaterial is based on radical polymerizable monomers, the polymeric hostmaterial contains equal to or less than 5 ppm radical initiator basedmaterial relative to the total weight of the polymeric host material,and a portion of the light converter is reflective.
 12. The device ofclaim 11 wherein the portion of the light converter that is reflectiveforms a wall of a light mixing chamber.
 13. The device of claim 12wherein the light mixing chamber comprises an exit window, wherein theexit window is a portion of the light converter that is transmissive.14. The device of claim 11 wherein the light source light is blue andthe visible converter light is one of yellow, green, and red.
 15. Thedevice of claim 11 wherein substantially all of the light source lightis converted by the light converter, such that light exiting the deviceis substantially all visible converter light.
 16. A device comprising: alight source for generating light source light, a light converter forconverting at least part of the light source light into visibleconverter light, the light converter comprising: a polymeric hostmaterial; and light converter nanoparticles embedded in the polymerichost material; wherein: the polymeric host material is based on radicalpolymerizable monomers, and the polymeric host material contains equalto or less than 5 ppm radical initiator based material relative to thetotal weight of the polymeric host material; and particles not havingnanoparticle character disposed in a path of one of the light sourcelight and the visible converter light.
 17. The device of claim 16wherein the particles not having nanoparticle character are micron sizedparticulate inorganic luminescent material.
 18. The device of claim 16wherein the particles not having nanoparticle character are reflectiveTi02 particles.
 19. The device of claim 16 wherein the light converteris enclosed by an encapsulation, wherein the encapsulation is configuredto reduce exposure of the light converter to
 02. 20. The device of claim16 wherein the polymeric host material is selected from the groupconsisting of a poly vinyl polymer, a poly acrylate polymer and athiol-ene polymer.